ci: fix macos-13 deprecation

[kevinjqliu]

Follow up to #3515, the correct replacement for macos-13 should be macos-15-intel for Intel-based architecture
according to
https://docs.github.com/en/actions/reference/runners/github-hosted-runners#standard-github-hosted-runners-for-public-repositories

I reverted the previous fix and did a find/replace for macos-13 -> macos-15-intel

[kevinjqliu]

@lidavidm please take a look!

[lidavidm]

Well, here I was hoping that I could point to GHA and say we have to drop x64 macOS, but I guess this works too 😅 thank you!

[lidavidm]

One of those jobs may actually not build, so I'll try to take a look...

[lidavidm]

I don't think this is related to this PR, we need to backport postgres/postgres@6da2ba1 as it appears suddenly it's misdetecting this (or rather detecting it now that it's macos-15 but then failing as it's not actually available for use). Let me merge this and then I can work on that bit.

snprintf.c:414:27: error: 'strchrnul' is only available on macOS 15.4 or newer [-Werror,-Wunguarded-availability-new]
  414 |                         const char *next_pct = strchrnul(format + 1, '%');
      |                                                ^~~~~~~~~
snprintf.c:366:14: note: 'strchrnul' has been marked as being introduced in macOS 15.4 here, but the deployment target is macOS 10.15.0
  366 | extern char *strchrnul(const char *s, int c);
      |              ^
snprintf.c:414:27: note: enclose 'strchrnul' in a __builtin_available check to silence this warning
  414 |                         const char *next_pct = strchrnul(format + 1, '%');
      |                                                ^~~~~~~~~
  415 | 
  416 |                         /* Dump literal data we just scanned over */
  417 |                         dostr(format, next_pct - format, target);
  418 |                         if (target->failed)
  419 |                                 break;
  420 | 
  421 |                         if (*next_pct == '\0')
  422 |                                 break;
  423 |                         format = next_pct;
      |